Among recent camera apparatuses, surveillance camera apparatuses are known which shoot surveillance video night and day. Such camera apparatuses use an IR cut filter (filter for color photography) for daylight photography and use optical glass corresponding to optical path length of the IR cut filter for night photography instead of using the IR cut filter. In the night, if there is axial chromatic aberration of a lens due to illumination or light containing infrared (IR) light, blurring will occur in the video when the IR cut filter is switched into and out of use. To deal with this, conventional camera apparatuses are equipped with an ABF (Auto Back Focus) mechanism which automatically adjusts focus by moving an image pickup element along an optical axis direction depending on whether an IR cut filter is used.
The ABF mechanism of conventional surveillance camera apparatuses uses a lead screw and stepping motor. The image pickup element is mounted on a nut rotatably engaged with the lead screw. The ABF mechanism moves the image pickup element forward and backward along the optical axis by rotating the lead screw using a rotational driving force of the stepping motor. For example, Japanese Patent Laid-Open No. 2003-274229 (pp. 3-5 and FIGS. 1 to 3) discloses such an ABF mechanism.
When size of the image pickup element is increased, a larger thrust force becomes necessary. However, with the ABF mechanism of the conventional camera apparatuses, when the rotational driving force of the stepping motor is converted into a thrust force along the optical axis, the resulting thrust force is not sufficiently large. To obtain a thrust force large enough to move the image pickup element along the optical axis, it is necessary to use a large stepping motor which develops a large rotational driving force. This makes it difficult to reduce the size and cost of the apparatuses. Also, the ABF mechanism of the conventional camera apparatuses finely adjusts an amount of travel along the optical axis by finely adjusting a rotation amount of the stepping motor, but there is a limit to the amount which can be adjusted using the stepping motor, making it difficult to more finely adjust the amount of travel.